mesa-buildtest: use llvm-config-4.0 for mesa >= 18.1
Created by: dcbaker
This is necessary to allow us to select different llvm-configs for different mesa branches. Namely for mesa >= 18.1 we need llvm 4.0, but for < 18.1 we still want to use 3.9
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Created by: dcbaker
This is necessary to allow us to select different llvm-configs for different mesa branches. Namely for mesa >= 18.1 we need llvm 4.0, but for < 18.1 we still want to use 3.9